1. Field of the Invention
The present invention relates to a phase adjuster and an encoder.
2. Description of Related Art
These days, encoders are used in order to detect a position of various devices having a driver. An encoder is a displacement position measuring device which can be attached to a driving shaft or a rotation shaft of a machine tool or a coordinate measuring apparatus. In general, an encoder may be a linear encoder for detecting linear displacement and a rotary encoder for detecting a rotational angle. Examples of a detecting method of an encoder include a magnetic type, a capacitive type, and an electromagnetic induction type in addition to a photoelectric type.
In a photoelectric encoder, there is a type which outputs a four-phase main signal including phase A (phase 0°), phase −A (phase 180°), phase B (phase 90°), and phase −B (phase 270°) in which each phase differs by 90° respectively. Hereafter, this main signal is referred to as a four-phase signal with 90° phase difference, or simply as a four-phase signal. By combining this signal with an origin point signal, the photoelectric encoder can output an ABS position (absolute position) with the accuracy of a minimum resolution of interpolation of the main signal.
In addition, in the four-phase signal noted above, the photoelectric encoder can generate a phase a (0°) by subtracting phase −A from phase A; and a phase b (90°) by subtracting phase −B from phase B. In such a case, the photoelectric encoder outputs a two-phase main signal. Hereafter, this main signal is referred to as a two-phase signal with 90° phase difference, or simply as a two-phase signal. Although the number of phases is different between the two-phase signal and the four-phase signal, the technical significance of the two is equivalent.
A shift may occur between a phase of the above-noted four-phase main signal and a phase of the origin point signal, for example, when the photoelectric encoder is attached to a device. As a result, there may be a case where a phase difference between the main signal and the origin point signal cannot be defined to a desired value. Accordingly, the phase difference is generally adjusted by mechanically making fine adjustments, in a movement direction of the encoder, to an attachment position of a detection head. However, since a pulse width of the origin point signal is narrow, adjustments are not possible while observing a size of the origin point signal. Therefore, completion of this adjustment process requires repeated trial and error and takes a long time.
In order to avoid taking a long period of time for the adjustment process, a technique has been suggested in which a phase of a main signal is displaced by a predetermined value by flipping of a switch; and synchronization is achieved between the main signal and an origin point pulse (Japanese Patent Laid-open Publication No. 2002-116060). In this approach, a frequency of trial and error can be reduced compared to the case noted above where fine adjustments are made mechanically. In addition, a technique has been suggested in which the phase adjustment of the main signal described above is automatically controlled by an operation of a CPU (Japanese Patent Laid-open Publication No. 2002-162253).
However, the inventors of the present invention have discovered an issue, discussed below, with respect to the above-noted techniques. According to the techniques in Japanese Patent Laid-open Publication Nos. 2002-116060 and 2002-162253, only rough phase adjustments are possible since a pitch of the phase adjustment is 90°. In such a case, depending on a state of the origin point signal, a situation may arise such that a proper synchronization between the main signal and the origin point signal cannot be performed after the phase adjustment.
FIG. 12 illustrates synchronizations of the main signal and the origin point signals when a pulse width of the origin point signal is large. Originally, the origin point signal preferably includes a pulse width of one cycle (2π) of a main signal MAIN, as shown in an ideal origin point signal ORG_IDEAL of FIG. 12. However, in reality, in the origin point signal, the pulse width varies and timing for rising and falling of the pulse may vary. For example, as shown in an origin point signal ORG1 of FIG. 12, although there is no movement in the origin point signal around a center of the pulse, the pulse width may have a larger pulse width than an ideal value (2π). In the example of the origin point signal ORG1, the pulse width is 3π. At this point in time, the rising position of the origin point signal ORG1 is −3π/2 and the falling position is 3π/2. In addition, an origin point signal ORG2 is defined in which the timing for rising and falling of the origin point signal is delayed by π/8.
A case is considered in which a phase adjustment with a step of 90° (π/2) width, as presented in Japanese Patent Laid-open Publication Nos. 2002-116060 and 2002-162253, is performed with respect to these origin point signals ORG1 and ORG2.
When the phases of the origin point signals ORG1 and ORG2 are delayed by 90° (π/2) and are set as origin points signals ORG10 and ORG20 respectively, as shown in FIG. 12, the origin point signal ORG20 can be understood as having activation states at two positions, phase 0 and phase 2π. In this case, the encoder may detect two origin points at phase 0 and phase 2π, resulting in a mistaken detection of the absolute value of the origin point.
Therefore, in order to prevent the occurrence of such mistaken detections, a technique is required so that the phase adjustment of a main signal which is smaller than 90° (n/2) can be performed.